Battery separator with Z-direction stability

ABSTRACT

A battery separator is a microporous membrane. The membrane has a major volume of a thermoplastic polymer and a minor volume of an inert particulate filler. The filler is dispersed throughout the polymer. The membrane exhibits a maximum Z-direction compression of 95% of the original membrane thickness. Alternatively, the battery separator is a microporous membrane having a TMA compression curve with a first substantially horizontal slope between ambient temperature and 125° C., a second substantially horizontal slope at greater than 225° C. The curve of the first slope has a lower % compression than the curve of the second slope. The curve of the second slope is not less than 5% compression. The TMA compression curve is graphed so that the Y-axis represents % compression from original thickness and the X-axis represents temperature.

BACKGROUND OF THE INVENTION

The use of microporous membranes as battery separators is known. Forexample, microporous membranes are used as battery separators inlithium-ion batteries. Such separators may be single layered ormulti-layered thin films made of polyolefins. These separators oftenhave a ‘shut-down’ property such that when the temperature of thebattery reaches a predetermined temperature, the pores of the membraneclose and thereby prevent the flow of ions between the electrodes of thebattery. Increasing temperature in the battery may be caused by internalshorting, i.e., physical contact of the anode and cathode. The physicalcontact may be caused by, for example, physical damage to the battery,damage to the separator during battery manufacture, dendrite growth,excessive charging, and the like. As such, the separator, a thin (e.g.,typically about 8-25 microns thickness) microporous membrane, must havegood dimensional stability.

Dimensional stability, as it applies to battery separators, refers tothe ability of the separator not to shrink or not to excessively shrinkas a result of exposure to elevated temperatures. This shrinkage isobserved in the X and Y axes of the planar film. This term has not, todate, referred to the Z-direction dimensional stability.

Puncture strength, as it applies to battery separators, is the film'sability to resist puncture in the Z-direction. Puncture strength ismeasured by observing the force necessary to pierce a membrane with amoving needle of known physical dimensions.

To date, nothing has been done to improve the Z-direction dimensionalstability of these battery separators. Z-direction refers to thethickness of the separator. A battery is tightly wound to maximize itsenergy density. Tightly winding means, for a cylindrically woundbattery, that forces are directed radially inward, causing a compressiveforce on the separator across its thickness dimension. In the increasingtemperature situation, as the material of the separator starts to flowand blind the pores, the electrodes of the battery may move toward oneanother. As they move closer to one another, the risk of physicalcontact increases. The contact of the electrodes must be avoided.

Accordingly, there is a need for a battery separator, particularly abattery separator for a lithium-ion battery, having improved Z-directionstability.

In the prior art, it is known to mix filler into a separator for alithium battery. In U.S. Pat. No. 4,650,730, a multi-layered batteryseparator is disclosed. The first layer, the ‘shut down’ layer, is anunfilled microporous membrane. The second layer, the dimensionallystable layer, is a particulate filled microporous layer. The secondlayer, in final form (i.e., after extraction of the plasticizer), has acomposition weight ratio of 7-35/50-93/0-15 forpolymer/filler/plasticizer. There is no mention of Z-directiondimensional stability; instead, dimensional stability refers to thelength and breadth dimensions of the separator. The filler is used as aprocessing aid so that the high molecular weight polymer can beefficiently extruded into a film. In U.S. Pat. No. 6,432,586, amulti-layered battery separator for a high-energy lithium battery isdisclosed. The separator has a first microporous membrane and a secondnonporous ceramic composite layer. The ceramic composite layer consistsof a matrix material and inorganic particles. The matrix material may beselected from the group of polyethylene oxide (PEO), polyvinylidenefluoride (PVDF), polytetrafluoroethylene (PTFE), polyurethane,polyarcylonitrile (PAN), polymethylmethacrylate (PMMA),polytetraethylene glycol diacrylate, copolymers thereof and mixturesthereof. The inorganic particles may be selected from the group ofsilicon dioxide (SiO₂), aluminum oxide (Al₂O₃), calcium carbonate(CaCO₃), titanium dioxide (TiO₂), SiS₂, SiPO₄, and the like. Theparticulate makes up about 5-80% by weight of the ceramic compositelayer, but most preferably 40-60%. There is no mention of Z-directionstability, and the particulate is chosen for its conductive properties.

SUMMARY OF THE INVENTION

A battery separator is a microporous membrane. The membrane has a majorvolume of a thermoplastic polymer and a minor volume of an inertparticulate filler. The filler is dispersed throughout the polymer. Themembrane exhibits a maximum Z-direction compression of 95% of theoriginal membrane thickness. Alternatively, the battery separator is amicroporous membrane having a TMA compression curve with a firstsubstantially horizontal slope between ambient temperature and 125° C.,a second substantially horizontal slope at greater than 225° C. Thecurve of the first slope has a lower % compression than the curve of thesecond slope. The curve of the second slope is not less than 10%compression. The TMA compression curve is graphed so that the Y-axisrepresents % compression from original thickness and the X-axisrepresents temperature.

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawing information about the preferred embodiment of the invention; itbeing understood, however, that this invention is not limited to theprecise information shown.

FIG. 1 is a graphical illustration of TMA compression curves for severaldiffering membranes.

FIG. 2 is a graphical illustration of TMA compression curves for severaldiffering membranes.

DESCRIPTION OF THE INVENTION

A battery separator, as used herein, refers to a thin, microporousmembrane that is placed between the electrodes of a battery. Itphysically separates the electrodes to prevent their contact, allowsions to pass through the pores between the electrodes during dischargingand charging, acts as a reservoir for the electrolyte, and may have a‘shut down’ function. Hereinafter, discussion of the battery separatorshall be made with reference to lithium-ion batteries, it beingunderstood, however, that the separator is not so limited.

Microporous membranes typically have porosities in the range of 20-80%,alternatively in the range of 28-60%. The average pores size is in therange of 0.02 to 2.0 microns, alternatively in the range of 0.04 to 0.25microns. The membrane has a Gurley Number in the range of 5 to 150 sec,alternatively 20 to 80 sec (Gurley Numbers refers to the time it takesfor 10 cc of air at 12.2 inches of water to pass through one square inchof membrane). The membrane may range in thickness from about 0.1 to 75microns, alternatively 8 to 25 microns. Membranes may be single layeredor multi-layered. In multi-layered membranes, at least one of themembranes will included the filler discussed in greater detail below. Amulti-layered separator may have three layers where the filled layer issandwiched between two other layers or two-filled layer may sandwichanother membrane. Other layer, as used herein, refers to any layer,including coatings, other than the inventive layer. Other configurationsare readily apparent to one of ordinary skill.

Thermoplastic polymer generally refers to any synthetic thermoplasticpolymer that softens when heated and returns to its original conditionwhen cooled. Such thermoplastic polymers include: polyolefins, polyvinylchlorides, nylons, fluorocarbons, polystyrenes, and the like. Of thethermoplastics, polyolefins are the most interesting. Polyolefinsinclude, but are not limited to, polyethylene, ultra high molecularweight polyethylene (not considered a thermoplastic by some, butincluded herein nevertheless), polypropylene, polybutene,polymethylpentene, polyisoprene, copolymers thereof, and blends thereof.Exemplary blends include, but are not limited to, blends containing twoor more of the following polyethylene, ultra high molecular weightpolyethylene, and polypropylene, as well as, blends of the foregoingwith copolymers such as ethylene-butene copolymer and ethylene-hexenecopolymer.

A major volume of thermoplastic polymer refers to a majority by volumeof the membrane being the polymer. A majority is greater than 50%,alternatively, 50 to 90%.

Inert particulate filler refers to any material that when uniformlyblended into the foregoing thermoplastic polymer does not interact norchemically react with the thermoplastic polymer to substantially alterits fundamental nature and will not, when used as a component of themembrane of a battery separator, have an adverse impact upon thechemistry of the battery. This filler may be any material that isthermally stable, i.e., maintains or substantially maintains itsphysical shape at temperatures above, for example, 200° C. Particulatemost often refers to a small bead or grain, but may also describe a flator planar object or a rod or fiber like object. The filler is small, andby small is meant an average particle size in the submicron (less than 1micron) range with a maximum particle size no larger than 40% of themembrane layer thickness, alternatively no larger than 10% of thelayer's thickness. In some applications (e.g., when making membraneswith a thickness of about 1 micron or less), filler with nano-sizedaverage particle sizes is beneficial.

Inert particulate filler may be selected from the following group ofmaterials: carbon based materials, metal oxides and hydroxides, metalcarbonates, minerals, synthetic and natural zeolites, cements,silicates, glass particles, sulfur-containing salts, synthetic polymers,and mixtures thereof. Exemplary carbon based materials include: carbonblack, coal dust, and graphite. Exemplary metal oxides and hydroxidesinclude those having such materials as silicon, aluminum, calcium,magnesium, barium, titanium, iron, zinc, and tin. Specific examplesinclude: TiO₂, MgO, SiO₂, Al₂O₃, SiS₂, SiPO₄. Exemplary metal carbonatesinclude those having such materials as: calcium and magnesium. Specificexamples include: CaCO₃. Exemplary minerals include: mica,montmorillonite, kaolinite, attapulgite, asbestos, talc, diatomaceousearth, and vermiculite. Exemplary cements include: portland cement.Exemplary silicates include: precipitated metal silicates (e.g., calciumsilicate and aluminum polysilicate), fumed silica, and alumina silicagels. Exemplary sulfur-containing salts include: molybdenum disulfide,zinc sulfide, and barium sulfate. Exemplary synthetic polymers include:polytetrafluoro ethylene (PTFE), polyimide (PIM), polyesters (e.g.,polyethylene terephtalate (PET)).

A minor volume of inert particulate filler refers to a minority byvolume of the membrane being the filler. A minority is less than 50%,alternatively 1-50%, or 5-45%.

The foregoing membranes may be made by any conventional process. The twomost widely used processes for making microporous membranes for batteryseparators are know as the dry-stretch (or Celgard) process and the wet(or extraction or TIPS) process. The major difference between theseprocesses is the method by which the microporous structure is formed. Inthe dry-stretch process, the pore structure is formed by stretching. Inthe wet process, the pore structure is formed by the extraction of acomponent. Both processes are similar in that the material componentsare mixed, typically in an extruder or via master-batching, and thenformed into a thin film precursor before pore formation.

The present invention may be manufactured by either process, so long asthe inert particulate filler is uniformly mixed into the thermoplasticpolymer prior to extrusion of the precursor.

In addition to the above combination of thermoplastic polymer andparticulate filler, the mixture may include conventional stabilizers,antioxidants, additives and processing aids as known to those skilled inthe art.

TMA (thermal mechanical analysis) measures the mechanical response of apolymer system as the temperature changes. The compression TMA measuresthe loss of thickness of a film when a constant force is applied in theZ-direction to the film as a function of increasing temperature. In thistest, a mechanical probe is used to apply a controlled force to aconstant area of the sample as the temperature is increased. Themovement of the probe is measured as a function of temperature. Thecompression TMA is used to measure the mechanical integrity of the film.

A standard TMA machine (Model No. TMA/SS/150C, Seiko Instruments Inc.,Paramus, N.J.) with a probe (quartz cylindrical probe, 3 mm diameter) isused. The load on the probe is 125 g. The temperature is increased atthe rate of 5° C./min. The film sample size is a single film with thedimensions of 5×5 mm.

In FIGS. 1 and 2, the X-axis represents temperature and the Y-axisrepresents % TMA. % TMA is percentage reduction in thickness of themembrane as a result of increasing temperature. For example, at 0° C.,the membrane's thickness is 100% under the specified load. In theinstant membrane, a maximum compression of 95% (or 5% of the originalthickness) is suitable to prevent electrode contact.

Referring to FIG. 1, there is shown four (4) TMA compression curves offour different membranes. Each membrane is a microporous membrane ofpolypropylene. Curve A is the control (i.e., no filler). Curve B has 4%by volume talc. Curve C has 8% talc. Curve D has 12% talc. Note that thecontrol has a maximum compression of 100% at 250° C., whereas Curves Cand D never cross the 80% compression lines.

Referring to FIG. 2, there is shown four (4) TMA compression curves offour different membranes. Each membrane is a microporous membrane ofpolypropylene. Curve A is the control (i.e., no filler). Curve B has2.5% by volume TiO₂. Curve C has 5% TiO₂. Curve D has 8.5% TiO₂. Notethat the control has a maximum compression of 100% at 250° C., whereasCurve B has a maximum compression of about 95% and Curves C and D have amaximum compression of about 90%.

The present invention may be embodied in other forms without departingfrom the spirit and the essential attributes thereof, and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicated the scope of the invention.

1. A battery separator comprising: a microporous membrane comprising amajor volume of a thermoplastic polymer and a minor volume of an inertparticulate filler, said filler being dispersed throughout said polymer,wherein said membrane exhibits a maximum Z-direction compression of 95%of the original membrane thickness.
 2. The battery separator of claim 1wherein said thermoplastic polymer being selected from the groupconsisting of: polyethylene, polypropylene, polybutene,polymethylpentene, ultrahigh molecular weight polyethylene, copolymersthereof, and blends of the foregoing.
 3. The battery separator of claim1 wherein said inert particulate filler being selected from the groupconsisting of: carbon based materials, metal oxides and hydroxides,metal carbonates, minerals, synthetic and natural zeolites, cements,silicates, glass particles, sulfur-containing salts, synthetic polymers,and mixtures thereof.
 4. The battery separator of claim 1 wherein saidmajor volume of said thermoplastic polymer being 50-90% by volume ofsaid membrane.
 5. The battery separator of claim 1 wherein said minorvolume of said inert particulate filler being 1-50% by volume of saidmembrane.
 6. The battery separator of claim 5 wherein said minor volumeof said inert particulate filler being 5-45% by volume of said membrane.7. The battery separator of claim 1 wherein said membrane exhibits amaximum Z-direction compression of 85% of the original membranethickness.
 8. A battery separator comprising: a microporous membranehaving a TMA compression curve with a first substantially horizontalslope between ambient temperature and 125° C., a second substantiallyhorizontal slope at greater than 225° C., wherein a Y-axis represents %compression from original thickness and a X-axis represents temperature,said curve of said first slope having a lower % compression than saidcurve of said second slope, and said curve of said second slope notbeing less than 5% compression.